1. Field of the Invention
The present invention relates to a carrier for carrying semiconductor wafers in a semiconductor device fabricating system, and a method of detecting wafers carried on a carrier.
2. Description of the Related Art
A carrier is used for carrying semiconductor wafers between semiconductor device fabricating equipment and inspection equipment. The carrier contains a predetermined number of wafers, such as thirteen or twenty-five wafers. Wafers are contained in carriers in lots, and are carried between the equipment in lots.
Wafer map of the wafers contained in the carrier are produced when subjecting the wafers to various processes. A semiconductor device fabricating system makes reference to the wafer map when subjecting the wafers to various processes, and records data about the results of the processes on the basis of the wafer map. When executing wafer mapping, it is necessary to detect the wafers stored in the slots of the carrier. It is also necessary to know, prior to subjecting the wafers to a process, if all the wafers have been taken out of the carrier.
In recent years, there is a trend toward the prevalent use of 12 in. (300 mm) diameter wafers instead of 6 in. and 8 in. diameter wafers, and semiconductor device fabricating systems and carriers for use in processing 12 in. diameter wafers have been developed. Carriers for carrying 12 in. diameter wafers are classified roughly into open carriers, and carriers enclosed in a pod, such as a unified pod, and it is expected that the latter will be used prevalently in the future.
In addition to progressive increase in the diameter of wafers, the continued reduction of the minimum device dimension has realized hyperfine configurations of the minimum feature length of sub-quarter .mu.m. Accordingly, the importance of techniques for the preparation of ultraclean rooms, the automatic transportation of wafers and the reduction of spaces for the installation of semiconductor device fabricating equipment and inspection equipment for semiconductor device manufacturing plants has progressively increased. The wafer detector becomes more important if the automation of semiconductor device fabricating equipment is promoted to reduce semiconductor device fabricating processes requiring operators work. Increase in the diameter of wafers entails sharp increase in the price of wafers. Therefore, it become more important to process large wafers correctly without failure. Consequently, the necessity of correct wafer mapping for the recognition of wafers contained in a carrier and reliable detection of wafers remaining in the carrier is augmented.
Semiconductor device fabricating systems are provided with water detectors employing a light emitting device and a photoelectric device to detect wafers optically. Some wafer detector of a direct reception type is provided with a light emitting device and a photoelectric device disposed opposite to each other, for example, above and below, or on the right and the left side, respectively, of a carrier to detect wafers contained in the carrier when the carrier is disposed between the light emitting device and the light receiving device. The wafer detector of this type projects a light beam by the light emitting device toward the photoelectric device, and decide that any wafer does not exist in slots in the carrier if the light beam is received by the photoelectric device and that a wafer is left in a slot if the light beam projected by the light emitting device is unable to fall on the photoelectric device.
Some wafer detector of a reflection type is provided with a light emitting device and a photoelectric device disposed side by side on one side of a carrier, and a reflecting mirror disposed on the opposite side of the carrier to detect wafers contained in the carrier when the carrier is disposed between the light emitting device and the light receiving device, and the reflecting mirror. The wafer detector of this type projects a light beam by the light emitting device toward the reflecting mirror, and decide that any wafer does not exist in slots in the carrier if the light beam reflected by the reflecting mirror is received by the photoelectric device and that a wafer is left in a slot if the light beam reflected by the reflecting mirror is not unable to fall on the photoelectric device.
The light emitting device and the photoelectric device are disposed above and below, or on the right and the left side, respectively, of a carrier transportation path or a carrier holding place in the former wafer detector, and the light emitting device and the photoelectric device, and the reflecting mirror are disposed above and below, or on the right and the left side, respectively, of a carrier transportation path or a carrier holding place in the latter wafer detector. Therefore, the water detector obstructs the transportation of the carrier. Such a wafer detector is disadvantageous in view of saving space because special spaces must be spared for the light emitting device, the photoelectric device and the reflecting mirror. The wafer detector of a reflection type is unable to discriminate between a light beam reflected from the reflecting mirror and a light beam reflected from a wafer and may possibly make faulty detection.